1. Field of the Invention and Contract Statement
The present invention relates to a method and apparatus for measuring very small torques developed along a rotating mechanical assembly.
The United States Government has rights in this invention pursuant to Contract No. DE-AC09-76SR00001 between the U.S. Department of Energy and E. I. DuPont de Nemours & Co.
2. Discussion of Background
Torque in a rotating shaft or mechanical assembly must sometimes be measured to a high degree of accuracy and without introducing significant extraneous force, elastic flexing or mechanical noise. For example, the development of gel strength as a function of time in such diverse products as jam and concrete is monitored by turning a set of blades at constant speed while measuring the resulting torque. Motion must be slow, as any disturbance may interfere with the gelling process; by the same token, vibration must be kept to a minimum. Since the torque developed is typically quite small, extraneous forces which could interfere with the measurement must also be minimized.
Most commonly, torque is measured using load-cell technology. A load cell consists of a section of elastic material which flexes to some degree in response to an applied force. The flexing is measured by one or more attached (or integral) resistive elements which carry an electric current; flexing alters their resistance, producing a small but measurable high-impedance electrical output proportional to the applied force. The majority of torque sensors are simply load cells so constructed that flexing takes place in response to torque, normally exerted between two end shafts or other connection points.
Sensors of this type must be used with caution. Mechanical "give" caused by the flexing motion may delay the attainment of equilibrium, slowing sensor response. Forces other than pure torque may cause extraneous flexing and false readings. Excessive torque (or force) may permanently deform the flexing element or cause the resistive elements to become detached or broken. To minimize these problems, flexing elements are commonly made of strong and stiff materials such as silicon and stainless steel. Since response is proportional to flexing, however, making a device more sensitive also makes it more vulnerable.
Additional complications arise when the sensor is part of a continuously rotating section of an assembly. Continuous rotation makes electrical connection difficult; wires forming a direct connection would quickly be twisted and broken. Alternative connection methods, such as the use of slip rings or electromagnetic coupling of various kinds, have been tried. All of these introduce some degree of error into the readings: slip rings and similar devices cause frictional noise that can obscure the very low-level strain gauge output, and electromagnetic methods increase the complexity and the expense of the circuitry and may have a tendency to drift.
Because of these problems, rotating torque sensors are used only rarely. When there is no alternative to measuring torque on a rotating shaft, differential gearing may be used to reverse the shaft's direction, while a stationary sensor measures the reactive torque on the gearbox. Because several moving parts are required, however, this method always introduces some degree of friction and mechanical noise. As a result, sensors of this type are generally not capable of accurate measurement in low torque ranges.